A typical nuclear magnetic resonance (NMR) facility, as illustrated in FIG. 1, includes probe 10 containing an analyte, i.e., sample, usually a chemical to be spectrally analyzed or tissue to be imaged. The analyte in probe 10 is subjected to a high intensity DC magnetic field, from a suitable source, and is excited by RF from transmitter 12. Probe 10 includes output coil 14 for supplying an analog signal to receiver 16. The analog signal typically has a center frequency between approximately 100 MHz-1 GHz and normally has a bandwidth where information about the analyte is contained of less than 5 MHz. Receiver 16 responds to the signal from probe 10 and excitation from transmitter 12 to derive digital signals representing "in phase" (I) and "quadrature phase" (Q) components representing the information in the signal derived from coil 14. Digital computer 18 responds to the I and Q representing signals derived by receiver 16 to drive output device 20, such as displays for amplitude versus frequency plots of the chemical sample spectrum or images of the samples.
Receiver 16 includes analog mixer 22, having a first input responsive to the output of probe 10 and a second input responsive to a constant frequency sine wave output of local oscillator 24, in turn synchronized by an output of transmitter 12. Mixer 22 derives an intermediate frequency (i.f.) that is passed by low pass filter 26, to the exclusion of other frequencies derived by the mixer. Low pass filter 26 is included in a suitable amplifier, not shown; in general, amplifiers of a type well known to those skilled in the art are not shown in FIG. 1 or the remaining figures of this document.
The i.f. output of low pass filter 26 is applied in parallel to I and Q processing channels 28 and 30. I and Q channels 28 and 30 are driven by a constant frequency sine wave output of local oscillator 32, synchronized by an output of transmitter 12. Local oscillator 32 drives analog mixers 34 and 36 of I channels 28 and 30, respectively, with mixer 36 being driven by the output of 90.degree. phase shifter 37. Mixers 34 and 36 respond to the inputs thereof to derive orthogonally phased I and Q channel signals including the information in the signal derived from coil 14 of probe 10. The output signals of mixers 34 and 36 are respectively applied to low pass filters 38 and 40, which pass the baseband frequencies and eliminate other output frequencies derived by the mixers to derive orthogonally phased I and Q channel baseband signals.
The I and Q baseband output signals of low pass filters 38 and 40 are respectively applied to analog to digital converters 42 and 44. Analog to digital converters 42 and 44 sample the baseband signals supplied to them by low pass filters 38 and 40 at a frequency controlled and synchronized by a reference time base or master clock also controlling transmitter 12. Each time converters 42 and 44 are supplied with a sampling pulse, the converters derive multi-bit outputs representing the amplitude of the signals supplied to them. To obtain the resolution necessary for accurate spectral analysis and/or imaging purposes, the output signals of converters 42 and 44 preferably include 12 to 18 parallel output bits, supplied to a 12-18 bit bus. The digital signals on the output buses of converters 42 and 44 are respectively supplied to digital computer 18.
A problem with receiver 16 of FIG. 1 is the use of analog components, such as mixers 34 and 36, low pass filters 46 and 48 and the amplifiers as well as the analog to digital converters associated therewith. These analog components must be closely matched to enable signals having the required accuracy to be supplied to analog to digital converters 42 and 44. In addition, as the components are subjected to different temperatures and age, there is a tendency for the analog components to derive signals that drift relative to each other. This causes inaccuracies in the relative values of the I and Q channel digital signals supplied to computer 18 and to output device 20.
It is, accordingly, an object of the invention to provide a new and improved nuclear magnetic resonance system, method and receiver having relatively low cost and great accuracy.
Another object of the invention is to provide a new and improved nuclear magnetic resonance system and method with an accurate, high resolution receiver that is relatively inexpensive and employs virtually all digital components.
Still another object of the present invention is to provide a new and improved nuclear magnetic resonance receiver for deriving in phase and quadrature phase channel signals by utilizing a single relatively inexpensive analog to digital converter for deriving a digital signal having high resolution, sufficient to provide accurate spectral and image data.